GEMOC ARC National Key CentreGriffin, W.L., Alard, O., Pearson, N.J. and O'Reilly, S.Y.
GEMOC Macquarie
Sulfide phases contain 80-100\% of the Re and Os in mantle-derived peridotites. Detailed petrographic studies coupled with in-situ LAM-ICPMS analysis of PGEs have identified two general types of mantle sulfides: (1) S-rich MSS with high Os (20-1000 ppm) but low Pd/Ir (0.001-1), typically occuring as inclusions in primary silicates; (2) S-poor, Ni±Cu-rich sulfides with low Os, Ir but high Pd/Ir (up to 1000), which typically occur interstitially or with metasomatic phases. LAM-MC-ICPMS techniques provide analyses of 187Os/188Os in single sulfide grains, with 0.1% (2se) precision for Os>100 ppm and 1-2% for Os=10-30 ppm; 187Re/188Os typically is measured with ca 5% precision. In-situ analysis of sulfides in peridotite xenoliths shows that the two types of sulfide differ significantly in their Re-Os systematics. Type 2 sulfides typically have 187Os/188Os ranging from asthenospheric (0.127) to highly radiogenic (0.175) values. Type 1 sulfides typically have less radiogenic Os, and may give realistic depletion ages; e.g. sulfide inclusions in olivine macrocrysts from Udachnaya kimberlite lie on an isochron with published analyses of sulfide inclusions in Udachnaya diamonds. Plots of 187Os/188Os vs 187Re/188Os for sulfides in single samples of spinel peridotites from basalts and garnet lherzolites from kimberlites define mixing lines that may have either positive or negative slopes. Both types of sulfide occur in many (most?) mantle peridotites. For example, enclosed sulfides in a Massif Central spinel peridotite have 187Os/188Os=0.115 (187Re/188Os=0.36) while metasomatic sulfides have 187Os/188Os=0.136 (187Re/188Os=0.001). Many published "depletion ages" for mantle-derived peridotites probably represent mixtures of sulfide generations and thus have doubtful age significance. Meaningful interpretation of Re-Os data in terms of mantle events requires understanding of the occurrence and mobility of sulfides in mantle peridotites. In-situ LAM-MC-ICPMS analysis is less precise than conventional techniques, except for high-Os sulfides. However, spatially resolved data greatly enhance the interpretation of depletion ages, and allow recognition of different sulfide generations, the mobility of sulfides and the processes that disturb Re-Os systematics in the lithospheric mantle.